Generally an ultrasound diagnostic apparatus not only displays an ultrasonographic image (a B-mode image) which visualizes a structure inside a bio-tissue based on a reflected wave that shows acoustic impedance distribution in tissue, but also has a function to visualize the blood flow and motion of tissues.
Also recently estimating the hardness of a tissue portion in use of ultrasound waves has begun, and data thus obtained is used for diagnosing tissue. A strain inside the tissue, generated by such a method as applying static pressure from the surface of the body or applying excitation using a relatively low frequency ultrasound waves, is measured using ultrasound waves, and hardness of the tissue is estimated based on the amount of the strain. To calculate this strain, the displacement distribution of each point, measured with the ultrasound waves, is divided by a distance between each point.
Upon calculating the motion of a tissue or hardness of a tissue portion like this, displacement measured with ultrasound waves is frequently used, and therefore in recent ultrasound diagnostic apparatuses, a critical technical issue is the displacement measurement using ultrasound waves.
Patent Literature 1 (PTL 1) discloses an ultrasound diagnostic apparatus that measures displacement using an echo signal string effectively focused on sampling points arranged on a hyperbola-ellipse. PTL 1 also discloses a technique for setting a virtual wave source and performing transmission using a plurality of elements in order to improve the SN ratio in the case of using an aperture synthesis method for effectively focusing the echo signals on sampling points arranged on a hyperbola-ellipse.